Spark plugs for use in internal combustion engines such as automobile engines have a spark plug insulator (may be referred to simply as “insulator” in the specification) formed from, for example, alumina-based sintered material, containing alumina (Al2O3) as a predominant component. The insulator is formed from such an alumina-based sintered material, since the alumina-based sintered material is excellent in heat resistance, mechanical strength, or the like. The alumina-based sintered material is formed by sintering a powder mixture containing a sintering aid including SiO2, MgO, or the like.
However, when a spark plug insulator formed from an alumina-based sintered material produced by sintering with a sintering aid, after sintering, the sintering aid may remain present to form a low-melting-point glass phase at a grain boundary of alumina crystal grains after sintering. In such a case, the low-melting-point glass phase is softened under high-temperature operation conditions of a spark plug; e.g., at about 700° C., possibly impairing withstand voltage characteristics of the insulator thereof.
Meanwhile, spark plugs have become to be employed not only with fossil fuels such as gasoline but also with a variety of fuels including biofuels (e.g., ethanol). Under such circumstances, spark plugs are operated in an acidic atmosphere or in a basic atmosphere and therefore, are required to have corrosion resistance to such an atmosphere.
Patent Document 1 discloses an invention whose object is to provide “a spark plug which is excellent in resistance to an acidic atmosphere (hereinafter may be referred to as acid resistance) and withstand voltage characteristics at high temperature” (paragraph: 0008). A characteristic feature of the spark plug disclosed in Document 1 resides in that the insulator of the spark plug “is formed of an alumina-based sintered material containing an Si component, a Ba component, a Ca component, and an Mg component so as to satisfy the following conditions (1) and (2), and substantially containing no B component” (claim 1). Condition 1 is as follows: the ratio RCa of the mass of the Ca component (as reduced to oxide) to the mass of the Si component (as reduced to oxide) is 0.05 to 0.40. Condition 2 is as follows: the ratio RMg of the mass of the Mg component (as reduced to oxide) to the sum of the mass of the mass of the Si component (as reduced to oxide), the mass of the Ca component (as reduced to oxide), and the mass of the Mg component (as reduced to oxide) is 0.01 to 0.08.
Patent Document 1 discloses that the spark plug has excellent resistance to an acidic atmosphere, but is silent on the resistance to a basic atmosphere.